The main units of the tailwheel undercarriage retracted into the engine nacelles, leaving their wheels partially exposed.

Three G.18s were put into service with Fiat's own airline, ALI, early in 1936; the feedback received was that the type was underpowered.

Fiat responded the following year with a revised version, the G.18V which had more powerful engines, and a redesigned fin and long dorsal strake.

In June 1940, ALI was brought under control of the Regia Aeronautica (Royal Italian Air Force), and the G.18s were put to use as transports.

This latter aircraft was involved in a major accident on 30 April 1944 when, loaded with munitions, it exploded on the runway at Bresso.